1. Field of the Invention
The present invention relates to a method of forming a fine resist pattern, and more particularly, to a method of forming a fine resist pattern of high resolution using a contrast enhancement layer.
2. Description of the Background Art
FIG. 6 is a sectional view of a substrate showing steps of conventional photolithography for forming a fine resist pattern.
Referring to FIG. 6(a), a positive type photoresist 1 is applied on a substrate 4. Referring to FIG. 6(b), light is directed selectively onto the positive type photoresist 1 using a mask 5. Referring to FIG. 6(c), the positive type photoresist 1 is developed. According to this method, there were problems of poor contrast and low resolution as shown in FIG. 6(c).
In order to solve such problems, a CEL technique (contrast enhancement photolithography) was developed.
FIG. 7 is a sectional view of a substrate showing various steps of a conventional CEL technique.
Referring to FIG. 7(a), a positive type photoresist 1 is applied on a substrate 4.
Referring to FIG. 7(b), a middle layer 2 is formed on the positive type photoresist 1. A contrast enhancement layer (referred to as CEL hereinafter) 6 is formed on the middle layer 2. The middle layer 2 serves to prevent mixing of the CEL 6 and the positive type photoresist 1. As disclosed in Japanese Patent Laying-Open No. 2-212851, the CEL 6 has great absorption with respect to the exposure wavelength prior to exposure, which is gradually reduced as exposure is carried out. That is to say, the CEL 6 is a layer containing material that increases the transmittance of light (called light fading dye component). Diazonium salt, stilbazolium salt, aryl-nitroso salt are known as light fading dye components. A phenol type resin is used as the coating formation component.
Referring to FIG. 7(c), light is selectively directed towards the positive type resist 1 having the CEL 6 applied using a mask 5.
Referring to FIGS. 7(c) and 7(d), the CEL 6 and the middle layer 2 are separated by water. Then, the positive type photoresist 1 is developed. According to this method, the CEL formed on the positive type photoresist 1 has the exposed portion thereof substantially transparent, so that the contrast between the exposed portion and the non-exposed portion is enhanced. Thus, a fine resist pattern of high resolution is obtained.
Although the above-described CEL technique allows the formation of a fine resist pattern of high resolution, there was the problem of variance in the dimension of the resist pattern when the resist film thickness changes, as shown in FIG. 8. This phenomenon is called the effect of multiple reflection in a film.
FIG. 9 shows a case where a resist 1 is applied on a substrate 4 having a stepped portion 4a as a typical example where there is a change in the thickness of the resist film.
Variation in the dimension of a resist pattern due to a change in resist film thickness is caused by the surface reflectance of the CEL being altered in accordance with the film thickness of the resist. This is because the amount of variation of the surface reflectance corresponds to the amount of variation of the exposure energy to the resist. The exposure energy to a resist is reduced as the surface reflectance is increased and increases as the surface reflectance is reduced. This means that the dimension of a resist pattern will not be changed by variation in the resist film thickness if the surface reflectance takes a constant value regardless of a change in the film thickness of a resist.
Calculation of surface reflectance will be described hereinafter.
FIG. 10 shows a light path in a multilayer film.
Referring to FIG. 10, a portion of light entering from the surface (Initial) repeats reflection and refraction infinitely to eventually exit from the surface of the CEL 6. The surface reflectance R is the ratio of energy (E.sub.1) of light entering the surface (Initial) to energy (E.sub.2) of the total light of the infinite light from the surface of the CEL 6, i.e. E.sub.2 /E.sub.1.
Because the calculation of the surface reflectance of a multilayer film is complicated, the calculation of the surface reflectance of a single layer film will first be described.
FIG. 11 shows a method of calculating the surface reflectance in the case where a resist is provided on a silicon substrate. In FIG. 11, n.sub.1, n.sub.2, and n.sub.3 represent indexes of refraction.
The surface reflectance R is represented by functions t, b, and .delta., as shown in the equation (1), where t represents the reflectance (Fresnel coefficient) at the n.sub.1 /n.sub.2 face (the top face of the resist) which is expressed by the equation (2). b represents the reflectance (Fresnel coefficient) at the n.sub.2 /n.sub.3 face (the bottom face of the resist) and is expressed by the equation (3). .delta. represents the change in phase in the resist and is expressed by the equation (4).
The surface reflectance of a multilayer film shown in FIG. 12 can be obtained by applying the basic method shown in FIG. 11.
Referring to FIG. 12, the surface reflectance of a multilayer film is obtained by the steps of calculating the surface reflectance (R.sub.1) at the interface between the resist and the middle layer, calculating the surface reflectance (R.sub.2) at the interface between the middle layer and the CEL on the basis of the surface reflectance R.sub.1, and calculating the surface reflectance (R.sub.total) at the surface of the CEL on the basis of the surface reflectance R.sub.2.
The method of calculating the surface reflectance of a multilayer film will be described more specifically hereinafter.
It is necessary to calculate the film thickness and the index of refraction respectively of the middle layer and the CEL material in calculating the surface reflectance of a multilayer film. Logically, these selections have a great degree of freedom.
Because the characteristic of light fading must be applied to the CEL material, the material thereof is limited. As a result, the selection of the index of refraction of the CEL material is generally fixed to approximately 1.7.
Therefore, the degree of freedom includes three types which is the index of refraction and the film thickness of the middle layer, and the film thickness of the CEL material.
FIG. 13 shows the change in the calculated value of the surface reflectance when the film thickness of the resist is varied in a multilayer film structure of the middle layer having an index of refraction and a film thickness of 1.5 and 1000.ANG., respectively, and CEL material having a film thickness of 1500.ANG. (index of refraction is 1.66), for example. Referring to FIG. 13, the abscissa indicates the film thickness of the resist, and the ordinate represents the surface reflectance. In FIG. 13, S.W.H (Standing Wave Height) represents the amplitude of the surface reflectance.
The S.W.H value can have the amplitude obtained from the relationship between the resist film thickness and the surface reflectance of FIG. 13. A great value of S.W.H indicates that the variation in the surface reflectance is great. A low S.W.H value indicates that the variation in the surface reflectance is small. A small change in the surface reflectance means that the surface reflectance is maintained substantially at a constant value even if there is a change in the film thickness of the resist, so that the dimension of the resist pattern is not easily varied by a change in the film thickness of the resist.